In medicine, a cathartic is a substance that accelerates defecation

Defecation

Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...

. This is in contrast to a laxative

Laxative

Laxatives are foods, compounds, or drugs taken to induce bowel movements or to loosen the stool, most often taken to treat constipation. Certain stimulant, lubricant, and saline laxatives are used to evacuate the colon for rectal and/or bowel examinations, and may be supplemented by enemas under...

, which is a substance which eases defecation, usually by softening feces

Feces

Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

. It is possible for a substance to be both a laxative and a cathartic. However, agents such as psyllium seed husks

Psyllium seed husks

Psyllium seed husks also known as ispaghula, isabgol, or psyllium, are portions of the seeds of the plant Plantago ovata, , a native of India. They are soluble in water, expanding and becoming mucilaginous when wet...

 increase the bulk of the feces.

Cathartics such as sorbitol

Sorbitol

Sorbitol, also known as glucitol, Sorbogem® and Sorbo®, is a sugar alcohol that the human body metabolizes slowly. It can be obtained by reduction of glucose, changing the aldehyde group to a hydroxyl group. Sorbitol is found in apples, pears, peaches, and prunes...

, magnesium citrate

Magnesium citrate

Magnesium citrate, a magnesium salt of citric acid, is a chemical agent used medicinally as a saline laxative and to completely empty the bowel prior to a major surgery or colonoscopy. It is available without a prescription, both as a generic brand or under the brand name Citromag or Citroma. It is...

, magnesium sulfate

Magnesium sulfate

Magnesium sulfate is a chemical compound containing magnesium, sulfur and oxygen, with the formula MgSO4. It is often encountered as the heptahydrate epsomite , commonly called Epsom salt, from the town of Epsom in Surrey, England, where the salt was distilled from the springs that arise where the...

, or sodium sulfate

Sodium sulfate

Sodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...

 were previously used as a form of gastrointestinal decontamination following poison

Poison

In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....

ing via ingestion. They are no longer routinely recommended for poisonings. High-dose cathartics may be an effective means of ridding the lower gastrointestinal tract of toxins; however, they carry a risk of electrolyte imbalances and dehydration.

During the 1918 flu pandemic, cathartics were used in the Fort Lewis, WA area. An original report by Elizabeth J. Davies, a public heath nurse, mentions cathartics, pneumonia

Pneumonia

Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

 jackets and copious amount of drinks as treatments for influenza patients.

Blood is a cathartic. Gastrointestinal bleeding will cause diarrhea

Diarrhea

Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...

.